Solve 347 - Top K Frequent Elements (#10)

Author: terenceponce

README.md

@@ -12,3 +12,4 @@ This is a monorepo containing my solutions to LeetCode problems written in vario
 - 49 - Group Anagrams - [Notes](notes/0049_group_anagrams.md) | [Elixir](elixir/lib/solutions/0049_group_anagrams/group_anagrams.ex) | [Ruby](ruby/lib/solutions/0049_group_anagrams/group_anagrams.rb)
 - 217 - Contains Duplicate - [Notes](notes/0217_contains_duplicate.md) | [Elixir](elixir/lib/solutions/0217_contains_duplicate/contains_duplicate.ex) | [Ruby](ruby/lib/solutions/0217_contains_duplicate/contains_duplicate.rb)
 - 242 - Valid Anagram - [Notes](notes/0242_valid_anagram.md) | [Elixir](elixir/lib/solutions/0242_valid_anagram/valid_anagram.ex) | [Ruby](ruby/lib/solutions/0242_valid_anagram/valid_anagram.rb)
+- 347 - Top K Frequent Elements - [Notes](notes/0347_top_k_frequent_elements.md) | [Elixir](elixir/lib/solutions/0347_top_k_frequent_elements/top_k_frequent_elements.ex) | [Ruby](ruby/lib/solutions/0347_top_k_frequent_elements/top_k_frequent_elements.rb)

elixir/lib/solutions/0347_top_k_frequent_elements/top_k_frequent_elements.ex

@@ -0,0 +1,12 @@
+defmodule LeetCode.Solutions.TopKFrequentElements do
+  @moduledoc false
+
+  @spec call(nums :: [integer], k :: integer) :: [integer]
+  def call(nums, k) do
+    nums
+    |> Enum.frequencies()
+    |> Enum.sort(fn {_num1, x}, {_num2, y} -> x >= y end)
+    |> Enum.take(k)
+    |> Enum.map(fn {num, _frequency} -> num end)
+  end
+end

elixir/test/solutions/0347_top_k_frequent_elements/top_k_frequent_elements_test.exs

@@ -0,0 +1,13 @@
+defmodule LeetCode.Solutions.TopKFrequentElementsTest do
+  use ExUnit.Case, async: true
+
+  alias LeetCode.Solutions.TopKFrequentElements
+
+  test "Case 1 works" do
+    assert TopKFrequentElements.call([1, 1, 1, 2, 2, 3], 2) == [1, 2]
+  end
+
+  test "Case 2 works" do
+    assert TopKFrequentElements.call([1], 1) == [1]
+  end
+end

notes/0347_top_k_frequent_elements.md

@@ -0,0 +1,46 @@
+# Top K Frequent Elements
+
+**Link to Problem**: https://leetcode.com/problems/top-k-frequent-elements
+
+## Solutions
+
+- [Elixir](../elixir/lib/solutions/0347_top_k_frequent_elements/top_k_frequent_elements.ex)
+- [Ruby](../ruby/lib/solutions/0347_top_k_frequent_elements/top_k_frequent_elements.rb)
+
+## Description
+
+Given an integer array `nums` and an integer `k`, return the `k` most frequent elements. You may return the answer in **any order**.
+
+## Examples
+
+### Example 1
+
+```
+Input: nums = [1,1,1,2,2,3], k = 2
+Output: [1,2]
+```
+
+### Example 2
+
+```
+Input: nums = [1], k = 1
+Output: [1]
+```
+
+## Approach
+
+We produce a hash map of the frequencies of each element in the list/array. We then convert this hash map back into a list/array
+and we sort the list/array based on the frequency in descending order.
+
+Once the list/array has been sorted, we take the first `k` elements and we produce a new list/array containing the first element of
+the tuple/subarray.
+
+## Thoughts
+
+One thing I learned from this problem is how the sorting works for both Elixir and Ruby. Apparently, they work the same way. It's
+just that the syntax looks slightly different.
+
+For example, Ruby has the spaceship operator `<=>` while Elixir has to use `<=` or `>=`.
+
+Also, for Elixir, we can skip the step where we have to convert the hash map into a list because the `Enum` module already knows
+how to iterate through a hash map.

ruby/lib/solutions/0347_top_k_frequent_elements/top_k_frequent_elements.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+def top_k_frequent(nums, k_elements)
+  nums.tally
+      .to_a
+      .sort { |a, b| b[1] <=> a[1] }
+      .first(k_elements)
+      .map(&:first)
+end

ruby/test/solutions/0347_top_k_frequent_elements/top_k_frequent_elements_test.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+require_relative '../../../lib/solutions/0347_top_k_frequent_elements/top_k_frequent_elements'
+
+class TopKFrequentElementsTest < Minitest::Test
+  def test_case_1_works
+    assert_equal [1, 2], top_k_frequent([1, 1, 1, 2, 2, 3], 2)
+  end
+
+  def test_case_2_works
+    assert_equal [1], top_k_frequent([1], 1)
+  end
+end